Alloys and Their Uses – NDA Chemistry Notes

Pure metals are rarely used in engineering applications. Pure gold is too soft for jewellery. Pure iron rusts easily and is not strong enough for construction. Pure aluminium is too weak for aircraft. The solution is to mix metals together. The result is an alloy.

An alloy is a homogeneous mixture of two or more metals (or a metal and a non-metal) that has better properties than any of the individual components.

NDA asked this chapter through alloy composition identification, match-list questions pairing alloy names with their components, and NOT-correct statement traps about whether alloys contain silver or whether they are mixtures or compounds. The brass-bronze confusion is the single most tested alloy fact across NDA papers.

1. What is an Alloy?

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. The components are mixed in the molten state and allowed to solidify.

Key facts about alloys:

• Alloys are mixtures, NOT compounds
• They have variable composition (the ratio of components can vary within limits)
• Their properties are different and usually better than those of the pure metals
• They cannot be separated by ordinary physical methods once formed

Alloy is not a solution. Alloys are homogeneous mixtures of metals in solid state. [NDA 2022-I]

Why do we make alloys? Alloying improves:

• Hardness (steel is harder than pure iron)
• Strength (duralumin is stronger than pure aluminium)
• Corrosion resistance (stainless steel does not rust)
• Melting point (solder melts at a lower temperature than either tin or lead alone)
• Electrical resistance (nichrome has higher resistance than nickel or chromium alone)
• Appearance (German Silver looks like silver but contains no silver)

2. Master Alloy Table

Alloy	Composition	Key Property	Major Use
Brass	Cu + Zn	Hard, corrosion resistant, golden colour	Musical instruments, coins, utensils, scientific apparatus, cartridges
Bronze	Cu + Sn	Hard, wear resistant, historically important	Statues, medals, bells, ship propellers, coins (ancient)
Stainless Steel	Fe + Cr + Ni	Does not rust, Cr₂O₃ protective layer	Kitchen utensils, surgical instruments, cutlery
Solder	Pb + Sn	Low melting point (~183°C)	Joining electrical wires and circuit boards
Duralumin	Al + Cu + Mg + Mn	Lightweight yet strong	Aircraft frames, spacecraft, railway coaches
Nichrome	Ni + Cr	Very high electrical resistance, heat stable	Heating elements in toasters, electric heaters, irons
German Silver	Cu + Zn + Ni	Looks like silver, contains NO silver	Cutlery, ornaments, decorative items
Gun Metal	Cu + Sn + Zn	Hard, resists corrosion	Guns, gears, bearings, marine fittings
Alnico	Al + Ni + Co	Strong permanent magnet	Loudspeakers, electric motors, microphones
Magnalium	Al + Mg	Very lightweight	Aircraft structures, scientific balance pans
Dental Amalgam	Hg + Ag + Sn	Soft when mixed, hardens quickly	Dental fillings
Bell Metal	Cu + Sn (high Sn)	Resonant, sonorous	Bells, gongs

3. Brass vs Bronze

Both brass and bronze contain copper. The difference is in the second metal.

Brass = Copper + Zinc. Bronze = Copper + Tin.

Both brass and bronze are copper-containing alloys. Brass additionally contains Zinc while bronze additionally contains Tin. [NDA 2011-I]

Bronze is used to make statues and medals. Brass is used in utensils, scientific apparatus, and cartridges. [NDA 2011-I]

Bronze = Cu + Sn. [NDA 2022-II]

4. Solder

Solder = Lead (Pb) + Tin (Sn)

Solder has a low melting point (~183°C for the eutectic mixture). This allows it to melt and flow into joints without damaging the surrounding components. When it cools, it solidifies and makes a strong electrical and mechanical joint.

Solder is an alloy of Lead (Pb) and Tin (Sn). [NDA 2021-I]

Why low melting point matters for solder: It must melt below the melting point of the components being joined, otherwise it would melt the circuit components during soldering.

5. Stainless Steel

Stainless Steel = Iron (Fe) + Chromium (Cr) + Nickel (Ni)

Stainless steel does not rust because chromium reacts with atmospheric oxygen to form a thin, tough, self-repairing layer of Cr₂O₃ on the surface, preventing oxygen and moisture from reaching the iron beneath.

Why curd should NOT be stored in copper vessels: Curd (yoghurt) is acidic. Acids react with copper, producing copper compounds that can be toxic. Curd should be stored in stainless steel or plastic vessels, not copper. [NDA 2023-I]

6. Nichrome

Nichrome = Nickel (Ni) + Chromium (Cr)

Nichrome has very high electrical resistance AND does not oxidise at high temperatures. When current flows through a nichrome wire, it converts electrical energy to heat efficiently.

The heating coil in electric heaters, toasters, and electric irons is made of nichrome.

Why not copper for heating elements? Copper has very LOW resistance. It carries current without generating much heat. Nichrome has HIGH resistance. It generates heat efficiently.

7. Duralumin

Duralumin = Aluminium (Al) + Copper (Cu) + Magnesium (Mg) + Manganese (Mn)

Pure aluminium is too weak for structural use. Duralumin adds strength while keeping the weight very low. It has an excellent strength-to-weight ratio, essential for aircraft where every kilogram matters.

Duralumin is used in aircraft frames, spacecraft, and railway coaches.

8. German Silver

German Silver = Copper (Cu) + Zinc (Zn) + Nickel (Ni)

German Silver contains absolutely NO silver. It is called “silver” only because it looks like silver. It is an alloy of copper, zinc, and nickel.

German Silver is an alloy of copper, zinc and nickel.

9. Alnico

Alnico = Aluminium (Al) + Nickel (Ni) + Cobalt (Co)

The name ALNICO itself tells you the composition: AL = Aluminium | NI = Nickel | CO = Cobalt.

Alnico makes strong permanent magnets. Used in loudspeakers, electric motors, and microphones.

10. Dental Amalgam

Dental Amalgam = Mercury (Hg) + Silver (Ag) + Tin (Sn)

Mercury dissolves silver and tin to form an initially plastic mixture that can be shaped in a tooth cavity. It hardens within minutes as the metals react. Dental amalgam contains mercury, the only common everyday alloy that does.

11. Na-K Alloy

Sodium-Potassium alloy (NaK) remains liquid at room temperature and is an excellent heat conductor. It is used as a coolant to transfer heat in nuclear reactors. [NDA 2018-I]

12. Alloys are Mixtures: NOT Compounds

This distinction is directly asked in NDA:

Property	Alloy (Mixture)	Compound
Composition	Variable	Fixed
Separation possible?	Yes (physical methods in theory)	Only by chemical methods
Energy change on formation	None	Energy released or absorbed
Properties	Different from pure metals	Entirely new properties
Melting point	Range (not sharp)	Sharp, fixed

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